Orthogonal Frequency Division Multiplexing (OFDM) is a form of wireless multi-carrier modulation wherein carrier spacing is selected so that each subcarrier is orthogonal to the other subcarriers. This orthogonality avoids adjacent channel interference and prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high spectral efficiency, resiliency to Radio Frequency (RF) interference, and lower multi-path distortion.
In OFDM the subcarrier pulse used for transmission is chosen to be rectangular. This has the advantage that the task of pulse forming and modulation can be performed by a simple Inverse Discrete Fourier Transform (IDFT) which can be implemented very efficiently as an Inverse Fast Fourier Transform (IFFT). Therefore, the receiver only needs a FFT to reverse this operation.
Incoming serial data is first converted from serial to parallel and grouped into x bits each to form a complex number. The number x determines the signal constellation of the corresponding subcarrier, such as 16 Quadrature Amplitude Modulation. The complex number are modulated in a baseband fashion by the IFFT and converted back to serial data for transmission. A guard symbol is inserted between symbols to avoid inter-symbol interference (ISI) caused by multi-path distortion. The discrete symbols are converted to analog and low-pass filtered for RF up-conversion. The receiver then simply performs the inverse process of the transmitter.
The seminal article on OFDM is “Data Transmission by Frequency-Division Multiplexing Using the Discrete Fourier Transform”, by S. B. Weinstein and Paul M. Ebert in IEEE Transactions on Communication Technology, Vol. com-19, No. 5, October 1971.
OFDM forms the basis for the Digital Audio Broadcasting (DAB) standard in the European market as well as the basis for the global Asymmetric Digital Subscriber Line (ADSL) standard. Development is ongoing for wireless point-to-point and point-to-multipoint configurations for Wireless Local Area Networks using OFDM technology. In a supplement to the IEEE 802.11 standard, the IEEE 802.11 working group published IEEE 802.11a, which outlines the use of OFDM in the 5.8-GHz band.
In a packet communication system, data that is communicated is first grouped into packets of data, and the data packets, once formed, are then communicated, sometimes at discrete intervals. Once delivered to a receiving station, the information content of the data is ascertained by concatenating the information parts of the packets together. Packet communication systems generally make efficient use of communication channels as the communication channels need only to be allocated pursuant to a particular communication session only for the period during which the data packets are communicated. Packet communication channels are sometimes, therefore, shared communication channels that are shared by separate sets of communication stations between which separate communication services are concurrently effectuated.
A structured data format is set forth in the present promulgation of the operating specification. The data format of a data packet formed in conformity with standards, such as the WiMedia or ECMA-368/369, includes a preamble part and a payload part. Other packet communication systems analogously format data into packets that also include a preamble part and a payload part. The payload part of the packet contains the information that is to be communicated. That is to say, the payload part is non-determinative. Conversely, the preamble part of the data packet does not contain the informational content that is to be communicated but, rather, includes determinative data that is used for other purposes. In particular, the preamble part of an WiMedia or ECMA-368/369 packet preamble includes three parts, a packet sync sequence, a frame sync sequence, and a channel estimation sequence. The packet sync sequence is of a length of twenty-one OFDM (symbols), the frame sync sequence is of a length of three OFDM symbols, and the channel estimation sequence is of a length of six OFDM symbols. Collectively, the sequences are of a time length of 9,375 microseconds.
Of particular significance, the preamble also is used for channel estimation. The radio channel upon which the packet is communicated undergoes reflections and is otherwise distorted during its communication to the receiving station. To receive the transmitted data correctly, the receiving station must be provided with a good estimate of the channel to permit proper compensation to be made of the channel. The channel estimation sequence is a known waveform that tells the receiver what the channel looks like. From this known waveform, the receiver can properly compensate the channel to help decode the unknown data sequences.
Ultra-wideband (UWB) includes technology having a bandwidth larger than 500 MHz or 25 percent of a center frequency. Contemporary interest exists in development of wireless versions of serial technologies, such as universal serial bus (USB), capable of UWB transmission rates due to the proliferation of USB-adapted devices in various computational and media systems.
UWB systems spread transmit energy across a wide bandwidth, some of which is occupied by other licensed users. To abide by the rules of government regulatory bodies such as the Federal Communications Commission (FCC), UWB systems may require a method for automatic detection of these other users (“victim services”) of the band and then avoid transmitting over those users. This concept is commonly referred to as Detect and Avoid (DAA). As referred to herein, a victim service comprises transmissions of a device in a licensed band. The licensed band may be shared among non-licensed systems, such as UWB systems. Accordingly, the victim service may require preferential transmission rights when the device operating on the licensed spectrum contends with devices operating, at least in part, on the same spectrum in an unlicensed usage. More generally, a victim service may refer to any transmission of a device having a preferential spectrum usage right with respect to another device.
DAA utilizes an algorithm that is dependent on the time and frequency domain characteristics, power levels and bandwidths of the victim service, making it a difficult problem to solve. The presence of noise and multipath reflections and radio impairments such as DC offsets only increase the complexity of the problem. In addition, the regulations in different countries may require different criteria for DAA of victim services.
Unintended radiation (known as spurs) can also trigger these mechanisms and cause unnecessary avoidance and denial-of-service. In such cases, it is difficult to distinguish an interferer from a spur.
Narrowband systems such as Bluetooth® have provided interference mitigation by using frequency hopping as a means of robustness to avoid interference from IEEE 802.11b systems that share the same unlicensed band. However, ultra-wideband systems occupy bandwidth involving several GHz and hence can interfere with multiple licensed services.
For OFDM-based UWB systems, state of the art techniques have proposed the use of the Fourier Fast Transform in order to detect the interferers if the interferer is seen above a certain detection threshold in the frequency domain. However, several issues complicate the detection process such as the time-varying nature of the victim service, the bandwidth, the effect of the victim service at null tones such as the DC tone or at band edges, the power level and spurs.
Therefore, it would be desirable to have a Detect and Avoid method that can handle the complexity of ultra-wideband interference and improve filtering of spurs that might trigger false alarm rejections.